1. Field of the Invention
This invention relates to smoke detectors. The invention relates more particularly to improvements in smoke detectors of the ionization type.
2. Description of the Prior Art
Ionization type smoke detectors are known and have been used as combustion product detectors in home and in industrial applications for early warning of fire. The ionization detector includes, generally, an ionization chamber having first and second electrodes, a means for establishing an electric field between these electrodes, and means for causing ionization of gaseous particles such as air particles within the chamber. In one form of detector, ionization is produced by exposing the air particles to a radioactive source located within the chamber. Charged particles comprising ions are produced by radiation and an ion current flows between the electrodes.
In an improved form of ionization smoke detector, the ionization chamber is formed by a first cylindrically shaped electrode body having a closure formed at one end thereof and by a printed circuit board and a base member which are positioned at an opposite end thereof. The second electrode comprises a disc shaped body which is supported within the ion chamber by a support means extending from the base body through an aperture in the printed circuit board and into the chamber. Electrical components including an integrated circuit chip and a relatively high reference resistance are also positioned within the chamber. This improved ionization chamber arrangement facilitates manufacture and reduces cost of the detector.
The support means for the second electrode includes a pillar shaped body which is integrally formed with the base member and extends through the circuit board. Since the second electrode is maintained at an electrical potential, the pillar and base members are fabricated of an electrically insulating material.
The smoke detector ionization chamber is a relatively high sensitivity apparatus wherein quiescent ion currents in the picoampere range flow between the first and second electrodes. The detection of combustion products is accomplished by sensing a reduction in the amplitude of the ion current. As indicated, this current is initially of low magnitude and its reduction in amplitude, accompanying the entry of combustion products into the chamber, is also relatively small. Accordingly, relatively high gain electronic detection and amplifying means are utilized with the smoke detector for sensing this variation in ion current amplitude. However, extraneous electric fields can operate to interfere with the proper operation of the detector. It is therefore desirable to shield the chamber and low signal level amplifying components from the interferring extraneous electrical sources which could cause false alarms. In the improved ionization smoke detector referred to above, electronic components which are affected by such extraneous electric influences are positioned within the chamber and the cylindrical electrode shields the chamber and these components from these fields.
It has been found that, at times, the operation of the ionization chamber is interfered with by fields penetrating apertures in the circuit board and which are established by electric charges accumulating on a surface of the detector. The charge accumulation can be, for example, triboelectric. Electric field penetration accompanies the extension of a body through an aperture in the circuit board. These fields, in view of the sensitivity of the ionization chamber and the relatively sensitive electronics utilized therewith, at times interfere with chamber operation and with the associated electronics.